1. The Field of the Invention
The present invention pertains to transvenous leads for use with implantable cardiac stimulators, and more particularly to a transvenous or catheter lead for use in defibrillating and pacing the heart and in sensing the response of the heart to the defibrillating and pacing stimuli.
2. The Prior Art
Various factors affect the human heart rate and contribute to changes of rate from what is termed the normal sinus rate range. In healthy persons, tachycardia (rates generally ranging in adults from 100 to 160 beats per minutes) is experienced as a result of such things as physical or emotional stress (exercise or excitement), consupmption of alcoholic or caffeinated beverages, cigarette smoking, or ingestion of certain drugs. Rates exceeding 200 pmb have been observed in younger persons during strenuous exercise.
Variation from normal sinus rate range is generally characterized as cardiac arrhythmia, and arrhythmia rates exceeding the upper end of the sinus rate range are termed tachyarrhythmias. Healthy persons usually experience a gradual return to the sinus rate after removal of the factors giving rise to sinus tachycardia.
Arrhythmias typically arise in the atria or ventricles as a consequence of an impairment of the heart's electro-physiologic properties such as excitability, conductivity, and automaticity (rhythmicity). Such arrhythmias require special treatment, and in some instances may require immediate emergency treatment toward preventing sudden death of the afflicted individual. For example, tachycardia may, in certain instances, lead to fibrillation of an affected chamber of the heart. During fibrillation, sections of conductive cardiac tissue of the affected chamber undergo completely uncoordinated random contractions, quickly resulting in a complete loss of synchronous contraction of the overall mass of tissue and a consequent loss of the blood-pumping capability of that chamber.
Because of the lack of contribution of the atrial chambers to cardiac output, atrial fibrillation is hemodynamically tolerated and not generally regarded as life-threatening. However, in the case of ventricular fibrillation, cardiac output ceases instantaneously as a result of the rapid, chaotic electrical and mechanical activity of the excitable myocardial tissue and the consequent ineffectual quivering of the ventricles. Unless cardiac output is restored almost immediately after the onset of ventricular fibrillation, tissue begins to die for lack of oxygenated blood, and death will occur within minutes.
Ventricular fibrillation frequently is triggered by acceleration of a centricular tachycardia. Hence, various methods and devices have been developed or proposed to treat and arrest the tachycardia before the onset of fibrillation. Conventional techniques for terminating tachycardia include pacing therapy and cardioverson. In the latter technique, the heart is shocked with one or more current or voltage pulses of generally considerably higher energy content than is delivered in pacing pulses. Unfortunately, the use of such therapy itself presents a considerable risk of precipitating fibrillation.
Defibrillation involves applying one or more high energy "countershocks" to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to re-establish an organized spreading of action potential from cell to cell of the myocardium, thereby restoring the synchronized contraction of the mass of tissue. If these chaotic contractions continue in any tissue section, the defibrillation may be short-lived in that the uncontrolled tissue section remains a potential source for reinitiating fibrillation of the entire mass. Successful defibrillation clearly requires the delivery of a shocking pulse containing a substantial amount of electrical energy to the heart of the afflicted person, at least adequate to terminate the fibrillation and to preclude an immediate re-emergence.
Typically, for transthoracic external defibrillation, paddles are positioned on the patient's thorax and from about 100 to about 400 joules of electrical energy is delivered to the chest area in the region of the heart. By the manner in which the shock is applied, only a portion of this energy is actually delivered to the heart and is available to arrest fibrillation. Where fibrillation occurs during oepn heart surgery, internal paddles may be applied to opposite surfaces of the ventricular myocardium and, in these instances, the energy required to be delivered is considerably less, on the order of 20 to 40 joules.
The pulse energy requirements for internal defibrillation with fully implantable defibrillators and electrode systems range from about 5 joules to approximately 40 joules. Of course, the actual energy level required may differ from patient to patient, and further depends on such factors as the type of pulse waveform and the electrode configuration employed. While advances and improvements in electrical energy sources in general, and pacemaker batteries in particular, have been made over the past few years, it is clear, nonetheless, that repeated delivery of amounts of energy at the higher end of that range from an implanted system will tend to deplete conventional batteries in relatively short order. Accordingly, reduction of the energy level required for internal defibrillation remains a key area of inquiry and investigation.
It is clear that the electrode configuration plays an important role in the amount of energy necessary to achieve successful defibrillation. An early U.S. Patent, in terms of the relative immaturity of developments in the field at the time, U.S. Pat. No. 2,985,172, issued in 1961, described a tissue contacting electrode for use in deliverying a high voltage discharge directly to the heart. Each electrical lead, the ring holding conductive foil members and enclosed in a gauze sock, with a flexible backing member at one side of the sock. The overall electrode pad was described as sufficiently flexible to assume a dished shape tightly engaging the tissue of the heart.
In U.S. Pat. No. 4,030,509, Heilman, et al. describe an implantable electrode system for ventricular defibrillation, in which the electrodes are arranged in a generally base-apex configuration with a split conformal base electrode positioned above the base of the ventricles in the region of the atria, and a cup-like conformal apex electrode positioned at the apex of the heart.
In U.S. Pat. Nos. 4,270,549 and 4,291,707, Heilman, et al. disclose defibrillation electrodes of rectangular shape designed for insertion through the soft tissues outside the pleural cavity for contacting the heart. Each electrode consists of a metallic mesh either sandwiched between two layers of inert electrical insulation material or backed with a single layer of such material stitched to the mesh.
In U.S. Patent No. 4,548,203, Tacker, et al. disclose an electrode system for use with implantable pulse generators employed for cardioversion or defibrillation. The system consists of two sets of opposed patch electrodes, one pair disposed laterally on the epicardium and the other pair disposed ventrally-dorsally, with each electrode orthogonal to the adjacent electrodes. The patent asserts that the presence of the latter pair of electrodes does not significantly alter the current distribution from the first pair, so long as the electrodes are relatively small with respect to the epicardial circumference and the two pairs are isolated from each other during current flow. The patent further ascribes the use of two pairs of electrodes implanted in spaced relationship as purportedly permitting the use of smaller electrodes, lower voltage and current, and lower total energy, with a more uniform current density and less hazard of damage to adjacent heart tissue, than had theretofore been achieved. Two current pulses are sequentially delivered to the separate pairs of electrodes to provide a temporal and spatial summation effect for the defibrillating current.
Nevertheless, such prior art electrode systems proposed for use with implantable defibrillators dissipate relatively large amounts of energy in delivering shocking pulses to the heart. A reduction in the shock strength required for defibrillation provides the advantages of reducing the size of the batteries required and, thus, the size of the implantable defibrillator, increasing battery life, and reducing the possibility of myocardial damage resulting from the shock. In U.S. patent application Ser. No. 019,670, filed Feb. 27, 1987, assigned to the same assignee as the present application, separate large area defibrillation patch electrodes are placed over the right ventricle and the left ventricle of the heart and secured directly to either the epicardium or the pericardium. Each electrode is fabricated from a conductive layer, such as a mesh, with a bio-compatible insulative backing layer overlying one side of the conductive mesh, and an electrically conductive connection between the mesh and a lead for delivering the fibrillating waveform from the implanted (or external) difibrillator to the electrode. Each electrode is contoured to conform to the shape of the heart in the region of the respective ventricle over which it is to be placed, and has a size and shape to encompass a substantial portion of the ventricular myocardium but not other areas of the heart.
Electrodes fabricated according to criteria set forth in the aforementioned commonly assigned application provide a more uniform potentional gradient field throughout the entire ventricular mass, lower electrode impedance, and substantially higher efficiencies in the transfer of electrical energy than had theretofore been achieved with prior art defibrillator electrodes. However, as is usual with plural patch electrodes, implantation involves thoracic surgery requiring opening of the patient's chest for epicardial or extrapericardial placement.
It is a principal object of the present invention to provide an improved defibrillator electrode system which is implantable without major thoracic surgery, and which achieves highly efficient transfer of energy.